Surface protective films of flexible printed circuits are for example adhesive-bonded polyimide films, called coverlay films, that are punched out with a die conforming to the pattern, and are screen-printed films of UV or heat curable overcoating agents having flexibility, with the latter being more advantageous particularly in workability. Known such curable overcoating resins include resin compositions based on epoxy resins, acrylic resins or mixtures thereof. These compositions are often based on resins modified by introduction of butadiene, siloxane, polycarbonate diol or long chain aliphatic structures, whereby the flexibility is improved and the warpage due to cure shrinkage or heat shrinkage is prevented while minimizing the reduction of heat resistance, chemical resistance and electrical insulating properties inherent to the surface protective films.
With recent weight reduction and miniaturization of electronic equipment, flexible substrates are reduced in weight and thickness and are therefore more significantly susceptible to the flexibility and cure shrinkage of the overcoating resin compositions. Consequently, the curable overcoating resins do not satisfy performance requirements in terms of flexibility and warpage due to cure shrinkage.
JP-A-2004-137370 (Patent Document 1) discloses a polyamideimide resin produced through reaction of a polycarbonate diol from a diol having 6 or less carbon atoms, with a diisocyanate compound to prepare a polyurethane having two isocyanate groups at both ends, and reaction of the diisocyanate-terminated polyurethane with trimellitic acid. Cured products of the resin have unsatisfactory long-term reliability in electrical characteristics.
Furthermore, when the tackiness of a surface protection film after thermal curing of a resin composition is large, for example, the surface protection film adheres to a tape substrate in winding a reel off in the TAB (Tape Automated Bonding) application, or adheres to a clamper of the film in the case of ILB (Inner Lead Bonding) of LCD driver. Particularly, in the case of a thermosetting resin which has good adhesion to a substrate, the tackiness is liable to remain after curing. On the other hand, if a resin composition which has poor adhesion to a substrate or metal is used, the surface protection film is liable to peel off from a substrate, or long-term reliability of electric insulation of the surface protection film is lowered.    Patent Document 1 JP-A-2004-137370